1. Field of the Invention
Embodiments of the present invention relate generally to semiconductor substrate processing and, more particularly, to systems and methods for cleaning native oxide and associated residue from a substrate.
2. Description of the Related Art
In the microfabrication of integrated circuits and other devices, electrical interconnect features, such as contacts, vias, and lines, are commonly constructed on a substrate using high aspect ratio apertures formed in a dielectric material. The presence of native oxides and other contaminants such as etch residue within these small apertures is highly undesirable, contributing to void formation during subsequent metallization of the aperture and increasing the electrical resistance of the interconnect feature.
A native oxide typically forms when a substrate surface, such as a bare silicon surface, is exposed to oxygen and water. Oxygen exposure occurs when substrates are moved between processing chambers at atmospheric or ambient conditions, or when a small amount of oxygen remains in a processing chamber. In addition, native oxides may result from contamination during etching processes. Native oxide films are usually very thin, for example between 5-20 angstroms, but thick enough to cause difficulties in subsequent fabrication processes. Therefore, a native oxide layer is typically undesirable and needs to be removed prior to subsequent fabrication processes.
For example, a particular problem arises when native silicon oxide films are formed on exposed silicon containing layers, especially during processing of metal oxide silicon field effect transistor (MOSFET) structures. Silicon oxide films are electrically insulating and are undesirable at interfaces with contact electrodes or interconnecting electrical pathways because they cause high electrical contact resistance. In MOSFET structures, the electrodes and interconnecting pathways include silicide layers formed by depositing a refractory metal on bare silicon and annealing the metal to produce a metal silicide layer. Native silicon oxide films at the interface between the silicon substrate and the deposited metal reduce the compositional uniformity of the silicide layer by impeding the diffusional chemical reaction that forms the metal silicide during anneal. This results in lower substrate yields and increased failure rates due to overheating at the electrical contacts. The native silicon oxide film can also prevent adhesion of layers which are subsequently deposited on the substrate.
Various techniques are known for removing native oxides from a surface prior to metallization, but generally have one or more drawbacks. Sputter etch processes have been used to reduce contaminants, but are generally only effective in large features or in small features having aspect ratios less than about 4:1. In addition, sputter etch processes can damage delicate silicon layers by physical bombardment. Wet etch processes utilizing hydrofluoric acid are used to remove native oxides, but are less effective in smaller features with aspect ratios exceeding 4:1. This is because the aqueous solution has difficulty penetrating and being removed from vias, contacts, or other small features formed on the substrate surface, resulting in incomplete removal of the native oxide film and subsequent contamination issues. Also used to remove native oxides is a cleaning plasma that is generated from a mixture of ammonia and nitrogen trifluoride gases. When condensed on a substrate surface with a native silicon oxide, the products of the cleaning plasma form a thin film, containing ammonium hexafluorosilicate, from the native oxide. The film can be subsequently heated and sublimed off of the substrate. In high aspect ratio features, however, the thin film may not completely sublime, leaving an unwanted residue on the substrate. Furthermore, water generated in the formation of the thin film may produce additional native oxide on the substrate.
Accordingly, there is a need in the art for methods of removing native oxides and associated residue from a substrate that does not have the disadvantages outlined above.